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Discover how populations function, from size and structure to factors affecting growth. Learn about limiting factors, survivorship curves, growth models, and human population trends.
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Population • A population is a group of organisms that belong to the same species and live in a particular place at the same time. • Populations can be examined in multiple ways • Populations can increase, decrease,or stay thesameover time
Properties of Populations • Population Size • A population’s size is the number of individuals that the population contains. • Population Density • Density is a measure of how crowded the population is. • Age Structure • A population’s agestructureindicates the percentage of individuals at each age.
Properties of Populations, cont. • Dispersion • Dispersion describes the distribution of individuals within the population and may be random, uniform, or clumped.
THINK What do you think might affect a population’s growth and size?
Key Point: A limiting factoris somethingthat keeps a population from growing infinitely Think: What are the 5 major limiting factors? Resources, Space, Predators, Disease, and Habitat There are 2 different types of limiting factors
1. Density-Independent Factors Key Point: Density-independent factors are not affected by the population size Do not depend on the number of individuals present in an area. Examples: Unusual weather, natural disasters, and seasonal cycles
2. Density-Dependent Factors • Key Point: Population-limiting factors, such as competition, are density-dependentbecause the effect on each individual dependsonthe number of other individuals present in the same area. Examples: Competition, disease, and predation
Survivorship Curves • Patterns of Mortality • Populations show three patterns of mortality or survivorshipcurves: • Type I(low mortality until late in life) • Type II (constant mortality throughout life) • Type III (high mortality early in life followed by low mortality for the remaining life span)
Survivorship Curves, cont. • Type I = large mammals (i.e., humans, elephants) • Type II = birds, lizards, rodents • Type III = fish, insects
Population Growth Rate • Scientists define the growthrateof a population as the amount by which a population’s size changes in a given time. • Population Size • Birth rate - death rate = growth rate If the birth rate increases, the population increases If the death rate increases, the population decreases
Exponential Growth • The exponentialmodeldescribes perpetual growth at a steady rate in a population • The model assumes constant birth and death rates • Assumes unlimited resources
Logistic Growth • In the logisticmodel, birth rates fall and death rates climb as the population grows & resources are consumed • When the carryingcapacity is reached, the number of individuals the environment can support is reached and population growth becomes stable
Human Population Growth • The Development of Agriculture • About 10,000 to 12,000 years ago, the development of agriculture increased the growth rate of the human population • The Population Explosion • Around 1650, improvements in hygiene, diet, and economic conditions further accelerated population growth • After World War II, the human population grew at the fastest rate in history, largely because of better sanitation and medical care in poorer countries
Human Population Growth • Population Growth Today • Today, developing countries have faster human population growth and lower standards of living than developed countries do • Although the overall growth rate has slowed, the world’s population is still increasing
Human Population Growth • Demographic Transition • Demography: The scientific study of human populations and how they change over time • Human populations have undergone rapid growth, yet in some developed countries, populations have stopped growing. • The demographic transition model help explains how these population changes happen
